The present invention relates to a control system for an electromagnetic clutch for a motor vehicle.
An automobile provided with a continuously variable belt-drive transmission with an electromagnetic clutch is disclosed in EP-A 151038. The electromagnetic clutch of the transmission is controlled by a control system to provide various operational modes for clutch torque (clutch current). One of the modes is selected in accordance with a position of a selector lever and driving conditions of the vehicle to control the electromagnetic clutch.
When starting the vehicle, the clutch is controlled by the clutch torque increasing in proportion to the engine speed. Slipping of the clutch when starting of the vehicle depends on the engine torque and the clutch torque. FIG. 7a shows a clutch torque curve Tc and engine torque curve Te at an opening degree of a throttle valve. When the clutch torque coincides with engine torque Te at a point A (stall speed Ns), the clutch begins to engage. At that time, as shown in FIG. 7b, until input speed of the clutch (engine speed Ne) coincides with output speed Nc thereof and the clutch is entirely engaged, at a point B, the engine speed is kept constant. Accordingly, the clutch slips between points A and B, where the clutch engagement rate E (Nc/Ne) is smaller than 1 (E&lt;1), which is the called stall condition.
However, the engine stall speed is influenced by variances in the clutch torque characteristic, which are caused by manufacturing tolerances and deterioration with time, or the reduction of engine torque at high altitudes.
If the engine torque reduces to a line Te' of FIG. 7a, the engine stall speed Ns decreases to a lower point A'. The vehicle starts at a small engine torque range so that the starting characteristic decreases. If the clutch torque reduces to a line Tc', the engine stall speed Ns increases to a point A", which causes a reduction of transmission efficiency and a rise of temperature of the clutch.